Spark Plugs = Bad Wiring Harness ?
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Newbie
Joined: Nov 2025
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2009 GL320
Thanks to posts like this one, I discovered before changing the spark plugs in my 2019 CLS450 (70k km) that the wiring harness in my car was in the same condition.
I had previously purchased a complete wiring harness to replace it along with the spark plugs.
Unfortunately, I wasn't aware that the connector has a constant 12V and GND. I unplugged two connectors from the coils to assess their condition, doing so with the ignition off. During the wiring harness replacement, the battery was disconnected, except for the coil connectors at the beginning... After replacing the entire wiring harness – it was a tough job, with limited access to some connectors and very little space – it turned out that when I started the car, it was running on only 3 cylinders.
Errors:
After checking the new engine wiring and connections, everything pointed to the ECU. On the working coils, one of the pins connected to the ECU had a 2-ohm resistance to ground. On the non-working ones, there was no connection.
I opened the ECU and traced the paths. It turns out that both cylinder groups (3 cylinders each) are interconnected. Three pins are connected by a thicker trace going to the PCB’s through-hole. There’s plenty of space, so I assume the manufacturer designed this part as a fuse that would burn out in case of a short circuit.
After soldering, the car started without any errors.
Additionally, I installed insulation on the intake manifold and the catalyst to isolate the coils. I mounted a fiberglass mat with aluminum foil. The installation doesn’t disturb the designed airflow, but it perfectly insulates against heat. After removing the engine cover, the temperature in that area is noticeably lower.
Regarding the flap in the front intake – I wouldn’t remove it. It naturally opens while driving. During a standstill, its absence would do more harm than good – it acts as a check valve. When the radiator fan is running, air is pushed through the engine bay towards the windshield. If we remove it, the lower resistance in that area will cause the air to blow in the opposite direction, instead of flowing over the engine.
I would suggest to all m256 owners to check conditions of the harness and replace it in advance to avoid ECU damage.
I had previously purchased a complete wiring harness to replace it along with the spark plugs.
Unfortunately, I wasn't aware that the connector has a constant 12V and GND. I unplugged two connectors from the coils to assess their condition, doing so with the ignition off. During the wiring harness replacement, the battery was disconnected, except for the coil connectors at the beginning... After replacing the entire wiring harness – it was a tough job, with limited access to some connectors and very little space – it turned out that when I started the car, it was running on only 3 cylinders.
Errors:
After checking the new engine wiring and connections, everything pointed to the ECU. On the working coils, one of the pins connected to the ECU had a 2-ohm resistance to ground. On the non-working ones, there was no connection.
I opened the ECU and traced the paths. It turns out that both cylinder groups (3 cylinders each) are interconnected. Three pins are connected by a thicker trace going to the PCB’s through-hole. There’s plenty of space, so I assume the manufacturer designed this part as a fuse that would burn out in case of a short circuit.
After soldering, the car started without any errors.Additionally, I installed insulation on the intake manifold and the catalyst to isolate the coils. I mounted a fiberglass mat with aluminum foil. The installation doesn’t disturb the designed airflow, but it perfectly insulates against heat. After removing the engine cover, the temperature in that area is noticeably lower.
Regarding the flap in the front intake – I wouldn’t remove it. It naturally opens while driving. During a standstill, its absence would do more harm than good – it acts as a check valve. When the radiator fan is running, air is pushed through the engine bay towards the windshield. If we remove it, the lower resistance in that area will cause the air to blow in the opposite direction, instead of flowing over the engine.
I would suggest to all m256 owners to check conditions of the harness and replace it in advance to avoid ECU damage.
Junior Member
Joined: Feb 2023
Posts: 16
Likes: 9
From: Haubstadt, IN
20 GLS450
Recently changed the plugs on my 2020 I6 and I was aware of the potential issue so I was extra cautious. The 2020 harness didn’t show any signs of deterioration, but I still handled it carefully. I did not have any issues other than removing plug #6.
Newbie
Joined: Apr 2018
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2013 gl450
SPARK PLUG REPLACEMENT ON GLS 450
I have replaced them on my 2020 gls450 at 60kmi. Not a complicated job except some dissasembly of the air intake pipes to acces the last couple plugs. Compared to my 2013 GL450 8cyl it is a much easier procedure. Just remove wires carefully and lubricate with a dielectric grease upon reassembly. Also, orient the plug electrodes properly.
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From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
Junior Member
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From: McCalla, Alabama
2013 CLS 550 Owner
I am considering buying a 3rd generation 2019 to 2023 CLS 450 with the M 256 engine. A lot of the available cars have less than 60,000 miles on them and the spark plug replacement hasn't come up yet. How prevalent is this problem? Did the wiring harness issue ever get resolved? Any advice will be welomed.
Junior Member
Joined: Nov 2024
Posts: 69
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From: Southern California
2021 CLS 53 AMG C257, 2023 W223 S580, 2021 X167 GLS450, 2022 V167 GLE350
I am considering buying a 3rd generation 2019 to 2023 CLS 450 with the M 256 engine. A lot of the available cars have less than 60,000 miles on them and the spark plug replacement hasn't come up yet. How prevalent is this problem? Did the wiring harness issue ever get resolved? Any advice will be welomed.
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MBWorld Fanatic!
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From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
Two things are interesting about your experience.
One - a spark plug harness failure can activate a check engine light.
Two - that an "extended warranty" can cover it.
A couple of questions, though - was this Extended Warranty one from the factory, specifically on the harness? Or was this an Extended Warranty that you purchased, like a CPO or factory ELW, or a Service Policy like Fidelity Platinum?
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From: U.S.
Current/Last 5-years:GLS;G70;Alpina B8;X7;Accord. Sold:X7,TeslaS;S560;S550,X5;530e;L.Navigator;LS460
Dealers are funny.
I did an oil change at around 50K miles, and dealer wanted me to also do the spark plugs. I declined and simply told him, nah thanks, am selling the car in a year and likely won't even hit the 60-65K. It's not worth it to do spark plugs for the next owner. He offered 20% discount on their 1200 USD estimate, and I declined again.
Later they called me, and said: Hey we will also replace your wiring harness (which will likely fail later when car is out of warranty) at no charge to you now (which is usually a 3500 USD job that warranty will pay us for). I said HOW? there is no check engine light, nothing. He said: Well, we are 100% sure that your wiring harness will break or will need to be replaced and can get the warranty to pay for our labor and the new harness, if you let us do the spark plug job (which I will pay for).
So I was like: YOU WANT ME TO FIND YOU A CAUSE TO GET PAID 3500 USD, when nothing is wrong today with my wiring harness, and the key to do is for me to accept and pay 1200 USD for a spark plug job that I don't feel I must do now. How about we split the 3500 USD in half, and you do spark plug for free, yet you would get the 3500 USD for the wiring harness (which according to him, he is confident he will get it covered by warranty, even without seeing it). To me, this feels like a scam. Dealers are scamming Mbenz, or maybe the wiring harness is just that bad that as soon as you touch it for a spark plug job, and it wall fall apart.
Well, I'd rather not worry about all this nonsense, and just drive my car to 60K miles or 65K miles. Am gonna sell it by then. Me doing spark plugs now means only this: 1. paying it for the next owner; 2. finding a cause for the dealer to get paid 3500 USD before my car is out of warranty. Am not getting any benefit unless am truly going to keep the car for 2-3 more years or many more miles. Remember folks: Dealers LOVE to find something wrong with your car and get it covered by a warranty claim to be paid by corporate
I did an oil change at around 50K miles, and dealer wanted me to also do the spark plugs. I declined and simply told him, nah thanks, am selling the car in a year and likely won't even hit the 60-65K. It's not worth it to do spark plugs for the next owner. He offered 20% discount on their 1200 USD estimate, and I declined again.
Later they called me, and said: Hey we will also replace your wiring harness (which will likely fail later when car is out of warranty) at no charge to you now (which is usually a 3500 USD job that warranty will pay us for). I said HOW? there is no check engine light, nothing. He said: Well, we are 100% sure that your wiring harness will break or will need to be replaced and can get the warranty to pay for our labor and the new harness, if you let us do the spark plug job (which I will pay for).
So I was like: YOU WANT ME TO FIND YOU A CAUSE TO GET PAID 3500 USD, when nothing is wrong today with my wiring harness, and the key to do is for me to accept and pay 1200 USD for a spark plug job that I don't feel I must do now. How about we split the 3500 USD in half, and you do spark plug for free, yet you would get the 3500 USD for the wiring harness (which according to him, he is confident he will get it covered by warranty, even without seeing it). To me, this feels like a scam. Dealers are scamming Mbenz, or maybe the wiring harness is just that bad that as soon as you touch it for a spark plug job, and it wall fall apart.
Well, I'd rather not worry about all this nonsense, and just drive my car to 60K miles or 65K miles. Am gonna sell it by then. Me doing spark plugs now means only this: 1. paying it for the next owner; 2. finding a cause for the dealer to get paid 3500 USD before my car is out of warranty. Am not getting any benefit unless am truly going to keep the car for 2-3 more years or many more miles. Remember folks: Dealers LOVE to find something wrong with your car and get it covered by a warranty claim to be paid by corporate
Last edited by S_W222; Feb 4, 2026 at 07:03 PM.
Junior Member
Joined: Sep 2018
Posts: 23
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From: McCalla, Alabama
2013 CLS 550 Owner
Thanks for your reply. I'm a big 2nd generation CLS fan and hated when the model line changed and then was discontimued with the 3rd generation. Perhaps it is best for me to steer clear of the 3rd generation CLS 450 models that have not corrected the wiring harness problem. Such beautiful cars, too bad they are not built to last past a reasonable period of time or mileage without something major happening.
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From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
Dealers are funny.
I did an oil change at around 50K miles, and dealer wanted me to also do the spark plugs. I declined and simply told him, nah thanks, am selling the car in a year and likely won't even hit the 60-65K. It's not worth it to do spark plugs for the next owner. He offered 20% discount on their 1200 USD estimate, and I declined again.
Later they called me, and said: Hey we will also replace your wiring harness (which will likely fail later when car is out of warranty) at no charge to you now (which is usually a 3500 USD job that warranty will pay us for). I said HOW? there is no check engine light, nothing. He said: Well, we are 100% sure that your wiring harness will break or will need to be replaced and can get the warranty to pay for our labor and the new harness, if you let us do the spark plug job (which I will pay for).
So I was like: YOU WANT ME TO FIND YOU A CAUSE TO GET PAID 3500 USD, when nothing is wrong today with my wiring harness, and the key to do is for me to accept and pay 1200 USD for a spark plug job that I don't feel I must do now. How about we split the 3500 USD in half, and you do spark plug for free, yet you would get the 3500 USD for the wiring harness (which according to him, he is confident he will get it covered by warranty, even without seeing it). To me, this feels like a scam. Dealers are scamming Mbenz, or maybe the wiring harness is just that bad that as soon as you touch it for a spark plug job, and it wall fall apart.
Well, I'd rather not worry about all this nonsense, and just drive my car to 60K miles or 65K miles. Am gonna sell it by then. Me doing spark plugs now means only this: 1. paying it for the next owner; 2. finding a cause for the dealer to get paid 3500 USD before my car is out of warranty. Am not getting any benefit unless am truly going to keep the car for 2-3 more years or many more miles. Remember folks: Dealers LOVE to find something wrong with your car and get it covered by a warranty claim to be paid by corporate
I did an oil change at around 50K miles, and dealer wanted me to also do the spark plugs. I declined and simply told him, nah thanks, am selling the car in a year and likely won't even hit the 60-65K. It's not worth it to do spark plugs for the next owner. He offered 20% discount on their 1200 USD estimate, and I declined again.
Later they called me, and said: Hey we will also replace your wiring harness (which will likely fail later when car is out of warranty) at no charge to you now (which is usually a 3500 USD job that warranty will pay us for). I said HOW? there is no check engine light, nothing. He said: Well, we are 100% sure that your wiring harness will break or will need to be replaced and can get the warranty to pay for our labor and the new harness, if you let us do the spark plug job (which I will pay for).
So I was like: YOU WANT ME TO FIND YOU A CAUSE TO GET PAID 3500 USD, when nothing is wrong today with my wiring harness, and the key to do is for me to accept and pay 1200 USD for a spark plug job that I don't feel I must do now. How about we split the 3500 USD in half, and you do spark plug for free, yet you would get the 3500 USD for the wiring harness (which according to him, he is confident he will get it covered by warranty, even without seeing it). To me, this feels like a scam. Dealers are scamming Mbenz, or maybe the wiring harness is just that bad that as soon as you touch it for a spark plug job, and it wall fall apart.
Well, I'd rather not worry about all this nonsense, and just drive my car to 60K miles or 65K miles. Am gonna sell it by then. Me doing spark plugs now means only this: 1. paying it for the next owner; 2. finding a cause for the dealer to get paid 3500 USD before my car is out of warranty. Am not getting any benefit unless am truly going to keep the car for 2-3 more years or many more miles. Remember folks: Dealers LOVE to find something wrong with your car and get it covered by a warranty claim to be paid by corporate
You can choose to do it or not, but in this case you might end up needing it unexpectedly, so why not take advantage of it?
It's a known issue. I'd be pissed if they didn't tell me about it.
My dealer did tell me about it at about 45,000 mi, they fixed it, and now I don't have to worry about my check engine light coming on and having to schedule another appointment.
It saved me time and money, and a bit of worry.
Last edited by mikapen; Feb 5, 2026 at 03:41 AM.
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Preventative maintenance.
You can choose to do it or not, but in this case you might end up needing it unexpectedly, so why not take advantage of it?
It's a known issue. I'd be pissed if they didn't tell me about it.
My dealer did tell me about it at about 45,000 mi, they fixed it, and now I don't have to worry about my check engine light coming on and having to schedule another appointment.
It saved me time and money, and a bit of worry.
You can choose to do it or not, but in this case you might end up needing it unexpectedly, so why not take advantage of it?
It's a known issue. I'd be pissed if they didn't tell me about it.
My dealer did tell me about it at about 45,000 mi, they fixed it, and now I don't have to worry about my check engine light coming on and having to schedule another appointment.
It saved me time and money, and a bit of worry.
MBWorld Fanatic!
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From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
Cause they want to charge me 1200 USD for something I wouldn’t need. They want me to be the one asking for a spark plug change job just so they can then squeeze in the harness warranty job and get paid 3500 usd by warranty in addition to my 1200 usd. I don’t need sparks as we will be trading in the car maybe next year and likely before 60-65k miles.
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Current/Last 5-years:GLS;G70;Alpina B8;X7;Accord. Sold:X7,TeslaS;S560;S550,X5;530e;L.Navigator;LS460
Junior Member
Joined: Sep 2018
Posts: 23
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From: McCalla, Alabama
2013 CLS 550 Owner
Thanks for confirming that the engine wiring is a known problem. I'm sure there is a way to take care of this and still enjoy these cars.
W_222 I would be one of the buyers who would be in the market for a CLS like yours. If you decide to sell yours, let me know.
W_222 I would be one of the buyers who would be in the market for a CLS like yours. If you decide to sell yours, let me know.
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From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
They could easily say "Hey we looked under the hood and it needs new wires!"
I'd probably stand there and raise a stink.
After asking them if they could look under the hood to see if it needs new wires.
On repair order: "Customer states that he needs new plug wires."
Last edited by mikapen; Feb 5, 2026 at 07:24 PM.
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That's not logical or acceptable.
They could easily say "Hey we looked under the hood and it needs new wires!"
I'd probably stand there and raise a stink.
After asking them if they could look under the hood to see if it needs new wires.
On repair order: "Customer states that he needs new plug wires."
They could easily say "Hey we looked under the hood and it needs new wires!"
I'd probably stand there and raise a stink.
After asking them if they could look under the hood to see if it needs new wires.
On repair order: "Customer states that he needs new plug wires."
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From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
@S_W222 I definitely agree in principle, and wouldn't participate in a shakedown either.
But I'll bet you do/will need a wiring harness.
My MB shop found them at 40k during a B service inspection. They just replaced them without asking.
But I'll bet you do/will need a wiring harness.
My MB shop found them at 40k during a B service inspection. They just replaced them without asking.
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2019 AMG GT 43 4door
I'm new to this forum and I really appreciate the information shared in this and other threads. It's a shame I didn't check here before changing the spark plugs.
I'm currently in a situation similar to many of you, and I mention @Owczar’s post because it describes symptoms very similar to what I'm experiencing.
I have a 2019 AMG GT43 4-door with 60000 km. I changed the spark plugs like I’ve done before (I already did this job around 30000 km). I disconnected the coils, removed them, replaced the spark plugs and reinstalled everything. When I start the car, everything seems normal, but after 2–3 seconds the check engine light comes on. At first I thought maybe I left a connector loose, so I checked everything quickly and started the car again. Same thing: after 2–3 seconds the check engine light appears.
I connected a diagnostic tool and it shows fault codes related to shorts. Looking more closely at the connectors, I noticed that the wires for coils 1, 2 and 3 were very cracked where they join the coil connector. I disconnected the battery, opened the harness, and all four wires for each coil had partially lost their insulation. As many of you mentioned, the insulation breaks down because of the heat.
While waiting for the new harness (10–15 days delivery), I rebuilt about 20–25 cm of wiring from each coil using new wires and new connectors After clearing the faults, the short circuit + low voltage errors come back even with just the ignition on. When starting the engine now, it runs fine for about 5–6 seconds, but then the check engine light comes back on again.
After reading this thread, I’m starting to suspect there might be a short inside the ECU. In @Owczar’s thread I can’t see the images of the ECU repair anymore. Could they be re-uploaded?Also, if the ECU is damaged, would it make sense that the engine initially runs fine for 2–3 seconds?
and after repairing the harness it now runs for about 5–6 seconds before the fault appears?
Thanks in advance.
Junior Member
Joined: Feb 2023
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From: Haubstadt, IN
20 GLS450
Just got my GLS450 back from the dealership after warranty work. Mileage at 46k, so I asked for a price quote on changing spark plugs (will be due at 50k). Price came back at $545. Not surprising...
What did surprise me was the note on the quote: PLEASE NOTE ENGINE WIRING HARNESS MAY ALSO NEED REPLACING FOR SPARK PLUG JOB TO BE SUCCESSFULLY COMPLETED, DEPENDING ON CONDITION OF HARNESS."
I asked my SA about the note, and he said "Oh, this engine has a known issue with the wiring harnesses; they often disintegrate during the spark plug job."
Normally, this wouldn't bother me, since my car is still under warranty... what do I care if they have to replace the wiring harness, right?
EXCEPT, I was toying with the idea of replacing the spark plugs myself, as an opportunity to teach my teen son some basic under-the-hood skills. Grrrr...
Anyone else with the 450 engine had a wiring harness replaced?
Thanks...
What did surprise me was the note on the quote: PLEASE NOTE ENGINE WIRING HARNESS MAY ALSO NEED REPLACING FOR SPARK PLUG JOB TO BE SUCCESSFULLY COMPLETED, DEPENDING ON CONDITION OF HARNESS."
I asked my SA about the note, and he said "Oh, this engine has a known issue with the wiring harnesses; they often disintegrate during the spark plug job."
Normally, this wouldn't bother me, since my car is still under warranty... what do I care if they have to replace the wiring harness, right?
EXCEPT, I was toying with the idea of replacing the spark plugs myself, as an opportunity to teach my teen son some basic under-the-hood skills. Grrrr...
Anyone else with the 450 engine had a wiring harness replaced?
Thanks...
Don't do it yourself, plugs 1-4 are easy, 5 is a bit of a pain, but number 6 requires a lot of disassembly around the turbo intake. I will not do it again, ever. I also read about the harness after I started, so I was very careful about how I handled it. I understand it is very pricey to replace. What I thought was going to be an afternoon's worth of work took about a week and I had StarDiag and WIS to assist. Not fun.
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2020 GLS450 (wife) / 2024 Ford Bronco (mine) / 2014 VW Tiquan (son)
I'm to my second round of spark plugs soon (100k miles). Changed them at 50k. But I'm not changing them anytime soon. Just not going to temp fate on that wiring harness. Car runs fine, nice and smooth, and 50k on spark plugs, in my opinion, is ridiculous. They should last a lot longer than that in a modern engine!
Just tossing in some fuel carbon cleaner here and there for awhile. I know won't have any impact on carbon, but may help keep the plugs clean for longer.
Just tossing in some fuel carbon cleaner here and there for awhile. I know won't have any impact on carbon, but may help keep the plugs clean for longer.
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From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
It's my understanding that the 50 or 60,000 mile spark plug requirement is based on the tendency for the threads to seize.
There's a lot of thread between the combustion chamber and the seal, so it kind of makes sense. I guess.
Techron is a good additive, since it's essentially what Chevron uses to make their fuels top tier plus.
There's a lot of thread between the combustion chamber and the seal, so it kind of makes sense. I guess.
Techron is a good additive, since it's essentially what Chevron uses to make their fuels top tier plus.
Last edited by mikapen; Mar 5, 2026 at 05:40 PM.
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So did Mbenz fix this issue with a new harness part number by the way? Or are they just installing the same harness again waiting for the same car to come back for a 4000 USD job after another 50K miles!?
